1. Field of the Invention
This invention generally relates to an improvement of ion-beam sputtering technology, and more particularly to an improvement of an apparatus and a method for performing a sputter deposition using an insulator as a target.
2. Description of the Related Art
Irradiating ions to bombard a target to cause an elastic or inelastic collision with atoms and molecules of a surface of the target for scattering the atoms/molecules or penetrating the irradiated ions into the target is generally called a sputtering process. Sputtering technology includes ion-beam sputtering, direct-current sputtering, and high-frequency sputtering.
Of these sputtering technologies, ion-beam sputtering employs an apparatus which comprises two chambers, i.e., an ion generating chamber where ion-beams are generated and a processing chamber where etching and/or film-forming or deposition is performed by irradiating the ion beams to the target.
The ion generating chamber is maintained at a low level of vacuum of at most 10.sup.-4 Torr and has on its ion beam emitting side a plurality of electrodes through which the ions are emitted. On the other hand, the processing chamber is kept at a high level of vacuum of about 10.sup.-5 Torr. In the processing chamber, a target and a substrate are arranged in position for processing.
With this ion-beam sputtering apparatus, since the processing chamber is maintained at a high level of vacuum as described above, a high-purity thin film can be obtained. And since no negative bias is applied to the target, as opposed to direct-current sputtering, an insulator can be used as the target.
However, when this conventional ion-beam sputtering apparatus is operated to perform ion-beam sputtering for 10 to 20 hours in total, employing as the target an insulator or a material which reacts with a gas in a high vacuum to produce an insulator, sputtered particles expelled from the target by ion beams adhere to all or part of the processing chamber wall surfaces or to all or part of the surfaces of the electrodes in the ion generating chamber, forming an undesired insulator film thereon.
As a result of this, when the surface of the so formed insulator film is subjected to ion irradiation, ions adhere to the film surface. Because the surfaces to which the ions adhere are insulative, the ions will not easily move. This causes local charges on the surface of the insulator film. Likewise the electrodes of the ion generating chamber are charged.
After the sputtering operation is continued for a long time, for example, for 10 or 20 hours, a potential difference between the locally charged portions and the uncharged portions increases, causing local discharge.
As a result, the ion beams might be curved, and the intensity of the ion beam to be irradiated on the target cannot be constant. This varies the processing conditions. The electrodes of the ion generating chamber are also subjected to a similar phenomenon since the electrodes are also covered with insulator film.
In order to solve this problem, after sputtering for about 10 to 20 hours in total, the high vacuum processing chamber is opened to the atmosphere, the wall surfaces of the processing chamber are cleaned, and the electrodes of the ion generating chamber are dismantled and then cleaned for a further operation.
When the conventional technology is used for effecting sputtering of an insulator substance for a long time of about 10 or more hours in total, the processing conditions such as the ion-beam intensity would vary for the reason as mentioned above, unless the cleaning etc. is carried out from time to time. In an etching operation, for example, the depth of the etching can not be constant.
As mentioned above, in order to make the processing conditions constant, the high vacuum processing chamber is opened to the atmosphere every 10 to 20 hours, and the wall surfaces of the processing chamber are cleaned, and further the electrodes of the ion generating chamber are dismantled and cleaned. This, however, lowers the efficiency of the operation.
Further, when the high vacuum processing chamber is opened to the atmosphere, moisture in the air is absorbed by the insulator film formed on the wall surface of the processing chamber. This absorbed moisture is not readily desorbed from the insulator film even after the processing chamber is evacuated. Thus, there is a fear that moisture will then be incorporated into the desired film.